[API reference](https://evolu.dev/docs/api-reference) › [@evolu/common](https://evolu.dev/docs/api-reference/common) › [Types](https://evolu.dev/docs/api-reference/common/Types) › Callback

```ts
type Callback<T> = (value: T) => void;
```

Defined in: [packages/common/src/Types.ts:22](https://github.com/evoluhq/evolu/blob/e7144e2bbe9069362b62dec1b64a8aa922b8f1b0/packages/common/src/Types.ts#L22)

A function that receives a value and returns nothing.

Use for event handlers, observers, and async completion handlers.

### Example

```ts
const onComplete: Callback<string> = (value) => console.log(value);
const queue = new Set<Callback<Result<Data, Error>>>();
```